Since polyesters such as polyethylene terephthalate have excellent physical and chemical properties, polyesters containing fine particles such as inorganic particles are widely used as fibers, films and other formed articles. Among these formed articles, films are used for magnetic recording media, capacitors, food packaging, heat sensitive mimeography and other general industries. For improving the handling convenience and the quality of these processed products, or being demanded for process passableness in the production or processing of films, it is generally practiced to add particles for making the surface of a film moderately rough or making the film or processed product thereof slippery or abrasion resistant.
The particles used generally include inorganic particles of silicon oxide, aluminum oxide, titanium oxide, calcium carbonate, barium sulfate, calcium phosphate, talc, kaolin, zeolite, calcium fluoride, lithium fluoride, molybdenum sulfide, etc., organic particles of silicone resin, fluorine resin, crosslinked polystyrene resin, crosslinked acrylic resin, etc., and also particles precipitated during polyester production.
Above all, salts such as carbonates and phosphates of elements of group IA and group IIA in the periodic table, especially phosphates are expected to be good in affinity to polyesters, because of their ionic bondability.
For example, the use of calcium phosphate particles is known in Japanese Patent Laid-Open (Kokai) Nos. 49-42752, 9-171939, etc.
As a means for improving calcium carbonate, Japanese Patent Laid-Open (Kokai) No. 10-1598 attempts to improve the running property, uniformity of surface protrusions and abrasion resistance of a film by controlling the volume average diameter, particle size distribution, content, etc. of the particles.
However, these conventional methods of using calcium phosphate particles, etc. are not satisfactory enough in the slipperiness and abrasion resistance of the obtained film. That is, it becomes difficult to achieve both sufficient slipperiness and abrasion resistance against production of shavings and coming-off of particles, when the film is passed at a high speed when produced or processed. The reason is considered to be that the affinity and adhesiveness between the particles and the polyester are still insufficient.
Especially in recent years in the area of magnetic recording tapes, higher image quality and higher density have been achieved, and polyester films are strongly required to be higher in slipperiness and abrasion resistance compared to those obtained by the conventional methods.
In addition, capacitor films are strongly required to have higher slipperiness, higher withstand voltage and higher insulation resistance in recent years while being reduced in thickness, though they have been improved in electric properties to some extent.
Furthermore, films for heat sensitive mimeographic paper are required to be excellent in perforability at low energy, uniformity in hole diameter after perforation and also in printing clearness.
Moreover, polyester films for lamination with a metallic sheet are required to be good in adhesiveness to the metallic sheet, free from the laminated film crystallization or deterioration, film peeling, shrinking, cracking or pinholes, etc. caused by heating after canning, and free from the film peeling or cracking caused by impact against the metallic can, and also required not to adsorb the flavor components of the contents of the can or not to impair the flavor of the contents by any component dissolved from the film or any odor emitted therefrom (hereinafter called flavor properties).